Upload
ganesa
View
70
Download
0
Embed Size (px)
DESCRIPTION
The Braidwood Neutrino Experiment. Ed Blucher, Chicago. Outstanding questions in neutrino oscillation physics: importance of 13 Experimental approaches to 13 ; motivation for a precise reactor experiment The Braidwood Experiment. Neutrino Oscillations. - PowerPoint PPT Presentation
Citation preview
17 November 2005 BNL HEP Seminar
1
The Braidwood Neutrino Experiment
• Outstanding questions in neutrino oscillation physics: importance of 13
• Experimental approaches to 13; motivation for a precise reactor experiment
• The Braidwood Experiment
Ed Blucher, Chicago
2Neutrino Oscillations
• During last few years, oscillations among different flavors of neutrinos have been established; physics beyond the S.M.
• Mass eigenstates and flavor eigenstates are not the same:
1 2 3 1
1 2 3 2
1 2 3 3
e e e eU U U
U U U
U U U
mass eigenstatesflavor eigenstates
• Raises many interesting questions including possibility of CP violation in neutrino oscillations.• CP violation in neutrino sector could be responsible for the matter-antimatter asymmetry
(leptogenesis)
MNSP matrix
( ) ( )N H N H The antilepton excess is converted to a baryon excess throughnonperturbative S.M. B+L violating, but B-L conserving processes.
3
2-Flavor Neutrino Mixing
1
2
cos sin
sin cose
The time evolution of the flavor states is:1 2
1 2
1 2
1 2
cos sin
sin cos
iE t iE te
iE t iE t
e e
e e
For a beam that is pure at t=0,
222 2
2 2 22 1
( ) sin 2 sin ,4
where m
e e
mP L
E
m m
4
What do we know?
Oscillations established with two distinct mass differences
1. Atmospheric: m2~2.5 eV2
Experiments using neutrinos produced by cosmic rays in atmosphere (e.g., SuperK); verified with long-baseline accelerator experiment (K2K).
Super Kamiokande
K2K
5
2. Solar: m2~55 eV2
Series of experiments using neutrinos from the Sun (e.g.,Ray Davis 37Cl experiment, SNO) and KAMLAND experiment using reactors in Japan.
Ray Davis SNO KAMLAND
6
Unconfirmed observation of oscillations with m2~1 eV2 by LSND does not fitinto 3 generation model (with 2 independent mass splittings).
MiniBoone should have results early nextyear.
What about LSND?
7
1 2
1 2 3
1 2 3
12 12 13 13
12 12 23 23
13
3
13 23
cos sin 0 cos 0 sin 1 0 0
sin cos 0 0 1 0 0 cos sin
0 0 1 sin 0 cos
?
0 sin co
CP
CP
ee e
i
i
U U Big Big
U U U U Big Big Big
U U U
U
Big Big Big
e
mall
e
S
23s
12 ~ 30° 23 ~ 45°
What is e component of 3 mass eigenstate?
Neutrino mixing and masses
normal inverted
sin2 213 < 0.15 at 90% CL
8
•What is value of 13?
•What is mass hierarchy?
•Do neutrino oscillations violate CP symmetry?
P( e ) P( e ) 16s12c12s13c132 s23c23 sin sin
m122
4EL
sin
m132
4 EL
sin
m232
4 EL
Value of 3 central to these questions; it sets the scale for experiments needed to resolve mass hierarchy and search for CP violation.
Key questions in neutrino mixing
•Why are quark and neutrino mixing matrices so different?
1
~ vs.
?
~ 1
1MNSP CKM
Big Big Small Small
U Big Big Big V
Sm
Small Small
Big Big Big Small Small
all
9Methods to measure sin2213
• Accelerators: Appearance (e) at m22.510-3 eV2
• Reactors: Disappearance (ee) at m22.510-3 eV2 2
2 2 1313( ) 1 sin 2 sin very small terms
4e e
m LP
E
22 2 2 213
23 13 13( ) sin sin 2 sin not small terms ( , ( ))4e CP
m LP sign m
E
Use reactors as a source of e (<E>~3.5 MeV) with a detector 1-2 kms awayand look for non-1/r2 behavior of the e rate
Reactor experiments provide the only clean measurement of sin22: no matter effects, no CP violation, almost no correlation with other parameters.
T2K: <E> = 0.7 GeV, L = 295 kmNOA: <E> = 2.3 GeV, L = 810 km
10
We recommend, as a high priority, a comprehensive U.S. program to completeour understanding of neutrino mixing, to determine the character of the neutrinomass spectrum, and to search for CP violation among neutrinos. This programshould have the following components:
• An expeditiously deployed multi-detector reactor experiment with sensitivity todisappearance down to sin22 = 0.01, an order of magnitude below presentlimits.
• A timely accelerator experiment with comparable sin22 = 0.01 sensitivity andsensitivity to the mass hierarchy through matter effects.
• A proton driver in the megawatt class or above and neutrino superbeam with anappropriate very large detector capable of observing CP violation and measuring the neutrino mass-squared differences and mixing parameters with high precision.
Recommendation 2 (of 3):
11
+/- 0.028
cp— normal— inverted
m2=2.510-3 eV2
Both reactor and accelerator experiments have sensitivity tosin22, but accelerator measurements have ambiguities
Example: T2K. P(e)=0.0045 sin2213=0.028
(5 yr
12
90% CL exluded regions with no osc.signal 90% CL allowed regions with osc.signal
Reactor and accelerator sensitivities to sin22
sin22θ13 = 0.05, δCP=0, Δm2 = 2.5×10-3 eV2
(3 yr reactor, 5 yr T2K)
δCP=0, Δm2 = 2.5×10-3 eV2
(3 yr reactor, 5 yr Nova)
Braidwood
13Resolving the 23 Degeneracy
Green: Nova OnlyBlue: Braidwood Reactor
plus Nova
Red: Double-Chooz plus offaxis disappearance experiments measure sin2223, while P(e)sin223sin2213.
•If 2345, disappearanceexperiments, leave a 2-fold degeneracy in 23 – it can be resolved by combination of a reactor and e appearance experiment. Braidwood (3 yrs) + Nova
Nova only (3yr + 3yr) Double Chooz (3yrs) + Nova
90% CL
Example:
sin22 23 = 0.95 0.01 Δm2 = 2.5×10-3 eV2 sin2213 = 0.05
Δm2 = 2.5×10-3 eV2 sin2213 = 0.05
14
CP
P( e
)
sin2213=0.1
T2K
CP
Nova
CP Violation and the Mass Hierarchy
15P
( e
)
CP
sin22=0.01from reactor
T2K - 5 years
Neutrino, normal hierarchy
Neutrino, inverted hierarchy
Example: Reactor + T2K running
sin2213=0.1
16
Nova and T2K Sensitivity to CP and Mass Hierarchy
If Braidwood does not see an oscillation signal, it will be difficult for
long-baseline “superbeam” experiments to investigate mass hierarchy and
CP violation.
17Reactor Measurements of Neutrino Oscillations
0.00
10.00
20.00
30.00
40.00
50.00
60.00
1.50 2.50 3.50 4.50 5.50 6.50 7.50 8.50 9.50
Enu (MeV)
Arb
itra
ry S
cale
Neutrino Flux Cross Section # of Events
Reactors are copious sources of per second.21: 5 GW 10 e e
Detection of antineutrino by e p e n
( 6 / fission)e
Flux
Cross
secti
onObservable spectrum
(~100 events /GW/ yr / ton at L= 1500 m)
18
2atmm 2
solarm2 2
2 2 2 213 1213 12( ) 1 sin 2 sin sin 2 sin
4 4e e
m L m LP
E E
Reactor Measurements of ( )e eP
13: Search for small oscillations at
1-2 km distance (corresponding to 2 ).atmm
Distance to reactor (m)
Pee
2 3 213
213
2.5 10
sin 2 0.04
3.5
m eV
E MeV
Past measurements:
Our sensitivity goal: sin22~0.01.Level at which long-baseline accelerator experiments can be used to measure mass hierarchy, CP violation.
19Chooz: Current Best Experiment
L=1.05 km
P=8.4 GWth
D=300mwe
m = 5 tons, Gd-loaded liquid scintillator
sin22< 0.15 for m2=2.5103 eV2
CHOOZ Systematic errors
Reactor flux
Detect. Acceptance
2%
1.5%
Total 2.7%
20
~200 m ~1300 m
How to improve on previous reactor experiments? Add an identical near detector
Eliminate dependence on reactor flux; only relative acceptance of detectors needed
Optimize baseline (1500 m) Larger detectors (5 ton 100 tons) Reduce backgrounds (Go deeper 100m 150 to 300 m; active veto systems)
21
Many sites have been discussed:
•Kraznoyarsk (Russia)•Chooz (France)•Kashiwazaki (Japan)•Diablo Canyon (California)•Braidwood, Byron (Illinois)•Wolf Creek (Kansas)•Brazil•Taiwan• Daya Bay (China)
22
Ref: Marteyamov et al, hep-ex/0211070
Reactor
Detector locations constrained by existing infrastructure
Features - underground reactor - existing infrastructure
~20000 ev/year~1.5 x 106 ev/year
Kr2Det: Reactor 13 Experiment at Krasnoyarsk
23
The Chooz site, Ardennes, France
24Daya Bay, China
25
U.S. Nuclear Power Plants
26
BraidwoodNeutrino
Experiment
Features of Braidwood Site:• 23.6 GW reactors – 7.17 GW maximum power• Flat: flexibility, equal overburden at near and far sites, surface transportation of detectors• Favorable geology (dolomitic limestone): good for excavation, low radioactivity (order of magnitude lower U, Th than granite)
Braidwood
27The Braidwood Collaboration
14 Institutions
70 Collaborators
28
Goals: Flexibility, redundancy, cross checks•4 identical 65 ton fiducial mass detectors; 2 at near site (L=270m), 2 at far site (L=1510m) •“Two zone detectors”: inner zone with Gd-loaded LS and r=2.6 m; outer zone with mineral oil and r=3.5 m.• Movable detectors with surface transport for cross-calibration; vertical shaft access to detector halls•Oscillation measurements using both rate and energy spectrum• Full detector construction above ground; detectorsfilled simultaneously with common scintillator.• Near and far detectors at same depth of 183 (464 mwe) gives equal spallation rates that can be exploited for detector and background checks
Braidwood Baseline Design
29
Near Near DetectorDetector
Far Far DetectorDetector
Braidwood Site
30Bore Hole Project at the Exelon Site
Bore hole project completed in January 2005
– Bore holes drilled to full depth (200m) at near and far shaft positions on Braidwood site.
– Provided detailed information on geology, ground water, radioactivity, etc.
– Confirmed feasibility of detectors down to depths of 460mwe.
– Reduces contingency required for underground construction
– Demonstrated willingness of Exelon to allow construction on their site.
31Braidwood Design Sensitivity
GOALS:
1. Discovery potential (at 3) for sin2213 > 0.01
2. Sensitivity (90% CL) down to the sin2213 = 0.005 level
With cross checks and redundancy to establish signal and check systematic errors
• See signal in both rate and energy spectrum measurements
• Cross calibrate detector pairs at high-rate near site
• Cross calibrate near/far detectors using spallation isotopes like 12B
• Multiple near and far detectors give direct cross checks on detector systematics at 0.05% for the near set and 0.3% for far
• Large detectors allow studies of the radial dependence of the IBD signal and backgrounds.
32
Normalization and spectral information
E (MeV)
E (MeV)
Predicted spectrum 13=0 (from near detector)
Observed spectrum (far detector)sin2213=0.04
Counting analysis: Compare number of events in near and far detectorSystematic uncertainties:• relative normalization of near and far detectors• relatively insensitive to energy calibration
Energy spectrum analysis: Compare energy distribution in near and far detectorsSystematic uncertainties:• energy scale and linearity• insensitive to relative efficiency of detectors
33Detectors and analysis strategy designed to minimize relative acceptance differences
6 meters
Shielding
To reduce backgrounds: depth + active and passive shielding
Central zone with Gd-loadedscintillator surrounded by bufferregions; fiducial mass determinedby volume of Gd-loaded scintillator
Events selected based on coincidenceof e+ signal (Evis>0.5 MeV) and s releasedfrom n+Gd capture (Evis>6 MeV).
No explicit requirement on reconstructedevent position; little sensitivity to E requirements.
,e p e n Neutrino detection by
n
e+
e
Gd-loaded liquidscintillator
n mGd → m+1Gd s (8 MeV); =20sec
34
– Outer steel buffer oil containment vessel (7m diameter)
• 1000 low activity glass 8” PMTs evenly distributed on inside surface (25% coverage)
– Inner acrylic Gd-loaded scinitillator containment vessel (5.2m diameter)
– Top access port can be used to insert calibration sources
Conceptual Mechanical Design
35Detector With Moveable Veto System and Shielding
36
Known volume of stable, identical Gd-loadedliquid scintillator in each detector
Well understood efficiency of positron and neutron energy requirements
Acceptance Issues
Must know: (relative) number of protons in fiducial region (relative) efficiency for detecting IBD events
37
Reconstructed Positron Energy
~E 0.8 MeV
Reconstructed NeutronCapture Energy
Reconstructed Energy Cuts:
• positron: Evis > 0.5 MeV
• n-Gd capture: Evis > 6 MeV
Monte Carlo Studies
Studies based on hit-levelsimulation with parameterizationsof many detector effects.
Studies using full GEANT4simulation are underway.
n Capture on H
n Capture on Gd
Reconstructed e+ and n-capture energy
38
Energy Scale
Use neutron capture peaks from IBD events to measure energy scale.
In each far detector, E scale can be measured to 0.3% every 5 days. (This calibration averages over detector in exactly the same way as signal events.)
Acceptance uncertainty from energy scale should be ~0.1%.
Calibration fromneutron capture peaks
0.1%uncertainty
39
I
IIIII
I. Gd-loaded liquid scintillatorII. catcher: liquid scintillator (no Gd)III. Non-scintillating buffer
3-zone versus 2-zone detectors
(Braidwood 2-zoneDesign)
40
Acceptance Sensitivity to Energy Scale
41
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.010
0.011
0.012
10/15/2004 12/4/2004 1/23/2005 3/14/2005 5/3/2005 6/22/2005 8/11/2005 9/30/2005 11/19/2005
Calendar Date
Ab
s a
t 4
30
nm
1.2% Gd in PC 0.2% Gd in PC (mixed samples from Gd16-Gd20)0.14% Gd in PC
0.2% Gd in 20%PC 80% Dodecane0.2% Gd in 40%PC 60% dodecane
338 days
297 days
259 days
198 days
Gd - Liquid Scintillator (Gd-LS)
• Detectors must be filled simultaneously common scintillator;
relative volume measurement with <0.2% uncertainty.
• We plan to use 0.2% Gd + 20% PC + 80% dodecane mixture
developed by BNL Nuclear Chemistry group.
(Dick Hahn, Minfeng Yeh, et al.)– Long-term stability tests in progress
– So far, stable with attenuation length > 18 m.
Stability of Gd-LS(Absorbance of 0.002 corresponds to attenuation Length of ~20 m).
Chooz degradation was 0.4%/day
x - Braidwood scintillator
42Movable Detectors
• Transport is necessary to move detectors from construction/filling area to below ground halls
• Movable detectors allow direct check of relative detector acceptances at
near site
• Possible scenario:
• Possible method: Use climbing jack system with cable to lift and put detectors on multi-wheeled trailer (standard method used in industry).
Goldhofer TrailerMoving 400 tons
Period Near Far
Initial 3 months
3 year data run
Final check
A B
A BC D
A CBD
43
• Unique feature of the Braidwood site: Near and far detectors have equal, well-understood,
substantial overburden
Can use produced 12B events to measure:• Near/far relative target mass from the total rate• Near/far energy calibrations from the relative energy
distribution
• ~50,000 12B beta-decay events per year per detector can be tagged and isolated giving a statistical uncertainty of 0.45%
• Systematic uncertainties related to the knowledge of relative near/far overburden; must be known to few percent from:– Geological survey information (Bore hole data: near/far agreement <1%)– Cosmic muon rates in the near and far locations
12BBeta decays t1/2 = 20ms (can tag to muon)13.4 MeV endpoint
Using Isotope Production to Measure Fiducial Mass
44
Summary of Acceptance Uncertainties
45Backgrounds
Even though near and far shielding is the same, backgrounds donot cancel: signal/background ratios in the near and far detectors are different.
– Uncorrelated backgrounds from random coincidences (not a problem)• Reduced by limiting radioactive materials
• Limestone rock at Braidwood site has low radioactivity
• Directly measured from rates and random trigger setups
– Correlated backgrounds
• Neutrons that mimic the coincidence signal
• Cosmogenically produced isotopes that decay to a beta and neutron
(9Li and 8He).
46Cosmic Muon Rates at Braidwood Depths
• Calculation of muon rate at 464 mwe (600 ft)– Used data from boreholes for density and
material– Average muon flux = 0.213 /m2/sec– Average muon energy = 110.1 GeV
Material Chemical composition Density (g/cm3)
Depth of top of layer (m)
Soil SiO2 1.60 0.0
Mudstone SiO2 2.46 11.3
Mudstone SiO2 2.52 27.1
Limestone CaCO3 2.61 42.7
Limestone CaCO3 2.63 61.0
Mudstone SiO2 2.60 63.1
Dolomitic Limestone 0.63* CaCO3 + 0.37*MgCO3
2.58 82.6
Dolomitic Limestone 0.63* CaCO3 + 0.37*MgCO3
2.62 98.8
Dolomitic Limestone 0.63* CaCO3 + 0.37*MgCO3
2.71 116.4
Dolomitic Limestone 0.63* CaCO3 + 0.37*MgCO3
2.62 135.0
Limestone CaCO3 2.63 142.3
Limestone CaCO3 2.71 157.6
Dolomitic Limestone 0.63* CaCO3 + 0.37*MgCO3
2.66 168.9
47Veto (Tagging) SystemGoal: < 1 n background event/day/detector.Strategy: tag muons that pass near the detector. Use shieldingto absorb neutrons produced by muons that miss the veto system.
Veto Detectors
p
n
n
6 meters
Shielding
Residual n background:
1. Veto inefficiency ─ 99% efficiency → 0.25/detector/day
2. Fast neutron created outside the shielding ─ 0.5/detector/day
With μ rate in the veto system of 21 Hz and the tag window of 100 μs → 0.2% dead time
Muon identification must allow in situ determination of the residual background rate
48Background Simulations
For a veto system with 2 mwe of
shielding, both a GEANT4 and a
MARS calculation give:
• 170 n/ton/day produced in the surrounding rock
• 4500 n/day emerging from the rock
• Background rate of ~0.75 events/ dayafter the veto requirements
Neutrons that reach the vessel wall
Fra
ctio
n o
f N
eu
tro
ns
Untaggedneutrons
De
tect
or
499Li and 8He
Isotopes like 9Li and 8He can be created in μ spallation on 12C and can decay to β+n.
Long lifetimes make veto difficult: 9Li~178ms
KAMLAND found isotope production correlated with muons that shower in the detector. from the thesis of Kevin McKinny
Tagging showering muons and rejecting events in a 0.5 s window eliminates 72% of 9Li and results in 7% deadtime.
Expect 0.078 9Li/ton/day; half decay in β+n modes; 72% are tagged 0.7/detector/day.
More…
50
Background Summary
Background Rate (<) Dead Time
Random 0.4/detector/day none
Fast Neutron (inside) 0.3/detector/day 0.2%
Fast Neutron (outside) 0.5/detector/day none9Li 0.7/detector/day 7%
Total 1.9/detector/day ~7%
Compare to 160 signal/detector/day at the far site (S/N~85)
51Sensitivity and Discovery Potential
• For three years of Braidwood dataand m2 > 2.5 x 10-3 eV2
90% CL limit at sin2213 < 0.005
3 discovery for sin2213 > 0.013
Summary of Uncertainties for 3 yr Data
With two near and two far detectors, thetotal uncertainty in the near/far ratio is 0.33%
5290% CL Sensitivity vs Years of Data
• Information from both counting and shape fits
• Combined sensitivity for sin2213 reaches the 0.005 after three years
0.001
0.010
0.100
1.000
0.1 1 10 100
Running Time (yrs)
sin
22q
13 9
0% C
L li
mit
(1.
28 s
)
count
shape
combine
3 ye
ars
Dm2 = 2.5e-3 eV2
53Braidwood Measurement Capability
For 3 years of data and a combined counting plus shape analysis m2 = 2.5 x 10-3 eV2 and sin2213 = 0.02
54Other Physics: Neutrino Electroweak Couplings
Braidwood experiment can isolate about 10,000e – e events that will allow the measurement of the neutrino gL
2 coupling to ~1%
– This is 4 better than past -e experiments and would give an error comparable to gL
2(NuTeV) = 0.3001 0.0014
Precision measurement possible since:– Measure elastic scattering relative to inverse beta decay
– Can pick a visible energy window (3-5 MeV) away from background
gL2 - gL
2(SM)
55
2004 Engineering/R&D proposal 2005 NuSAG Review2006 Full proposal submission2007 Project approval; construction start2010 Start datataking
Cost Estimate:
Civil Costs: $34M + $8.5M (Cont.)
4 Detectors and Veto Systems: $18M + $5M (Cont.)
Status of Project
Exelon enthusiasticsupporter of project
56
Conclusions
•The worldwide program to understand oscillations and determine the mixing parameters, CP violating effects, and mass hierarchy will require a broad range of measurements – a reactor experiment to measure 13 is a key part of this program.
•A reactor experiment will provide the most precise measurementof 13 or set the most restrictive limit.
•Reactor experiment with sensitivity of sin22~1% will give information needed to understand future roadmap of neutrino program.
•Braidwood offers an ideal site to perform an experiment with the required sensitivity (sin2213 = 0.005 at 90% c.l.)